Conversion of 2D MXene to Multi‐Low‐Dimensional GerMXene Superlattice Heterostructure

Abstract Integration of 2D structures into other low‐dimensional materials results in the development of distinct van der Waals heterostructures (vdWHSs) with enhanced properties. However, obtaining 2D–1D–0D vdWHSs of technologically useful next generation materials, transition‐metal carbide MXene and monoelemental Xene nanosheets in a single superlattice heterostructure is still challenging. Here, the fabrication of a new multidimensional superlattice heterostructure “GerMXene” from exfoliated M3X2T x MXene and hydrogenated germanane (GeH) crystals, is reported. Direct experimental evidence for conversion of hydrothermally activated titanium carbide MXene (A‐MXene) to GerMXene heterostructure through the rapid and spontaneous formation of titanium germanide (TiGe2 and Ti6Ge5) bonds, is provided. The obtained GerMXene heterostructure possesses enhanced surface properties, aqueous dispersibility, and Dirac signature of embedded GeH nanosheets as well as quantum dots. GerMXene exhibits functional bioactivity, electrical conductivity, and negative surface charge, paving ways for its applications in biomedical field, electronics, and energy storage.


Supplementary
: Schematic illustration of conversion of transition metal carbide MXene to multi-dimensional GerMXene superlattice heterostructure. a-f, The model depicts treatment of MXene to produce A-MXene complex. Purple, pink, black and beige colors represent titanium, aluminium, carbon and hydrogen respectively. Red and green colored balls represent the functional groups on A-MXene surface. g-j, Fabrication of monoelemental 0D GeH quantum dots from 2D hydrogenated GeH nanosheets (dark, light and ultralight blue marked the highest, lowest and other germanium atoms on GeH surface. k, van der Waals-covalent assembly of 2D-1D-0D GerMXene heterostructure anchored by raspberry-like nanoparticles at room temperature. Figure S2: Morphology of 2D Ti 3 C 2 T x MXene nanosheets and crystalline A-MXene complex. a, TEM images of accordion-like MXene nanosheets at different magnifications. b, TEM of A-MXene material, after autoclaving the material at 121 ºC for 30 minutes and bath sonication for 45 minutes. The obtained crystalline A-MXene composites are shown to possess higher stability in solid and aqueous media compared to pristine T 3 C 2 T x nanosheets. Figure S3: Diameter range of grown particles in the structure of A-MXene. a-b, The size distribution confirms the presence of two different types of anchored particles into Ti 3 C 2 T x nanocrystals. a, Ti 3 C 2 T x quantum dots and b, titanium oxide nanoparticles in the surface.

Supplementary Equations 1-4: Chemical reactions depicting phase-transformation of 2D Ti 3 C 2 T x MXene nanosheets to A-MXene complex.
Colloidal suspensions of titanium carbide MXene nanosheets were activated by hydrothermal treatment to form A-MXene crystals. The chemical reactions present the formation of a unique morphology of MXene materials that includes Ti 3 C 2 T x nanosheets, quantum dots and stable surface titanium oxide nanoparticles as described in the following equations. These reactions are in agreement with the previous reports on the structural change and phase transformation of 2D MXene in aqueous media 1 .
(1) 2Ti 3  Supplementary Figure   Supplementary Figure S11: Wall-to-wall interlayer measurement distribution of GerMXene superlattice heterostructure. The frequency of data showed that the interlayer distance of GerMXene was reduced compared to MXene. This phenomenon could be due to the higher secondary nucleation of GerMXene material and embedded quantum dots in its structure.
Supplementary Figure S12: Characterization of GerMXene heterostructure. Different magnification SEM images demonstrating the morphological details of the synthesized multidimensional GerMXene heterostructure.
Supplementary Figure S14: Phase stability prediction, estimated prototype and material properties computation of titanium germanide (TiGe 2 & Ti 6 Ge 5 ) bonds. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] In contrast with the TiGe 2 which includes Fddd space group in its crystallization structure, Ti 6 Ge 5 has a threedimensional structure with crystallization in the orthorhombic space group of Ibam. Supplementary Figure S26: Thermophysical properties and decomposition resistance of GerMXene superlattice heterostructure. Thermogravimetric analysis (TGA) of GerMXene powder is assessed and compared with Ti 3 C 2 T x MXene nanosheets. After annealing at a temperature up to 1000 °C under nitrogen atmosphere, the TGA analysis for GerMXene demonstrated no significant changes in the surface terminations or decomposition rate of the material. Furthermore, the TGA curves showed no significant mass loss with char residues of higher than 97%. Under nitrogen and temperature of above 600 °C conditions, the functional groups start to desorb from MXenes' surface, resulting in a minor mass loss (⁓ 2%). This signal is partially overlapped with the deprotonation of H 2 O during measurement. The samples were pre-heated in a vacuum oven at 60 °C for 12 hours to remove water content. These data demonstrate excellent thermal stability of GerMXene at tested temperatures.

Supplementary
Supplementary Figure S27: Morphology of precipitated GerMXene colloids after spinning at 1500 and 3000 rpm for 15 minutes. a,b, The SEM images showed that GerMXene material at concentration of 100 µg.mL −1 was highly stable after centrifugation at 1500 for 15 minutes. c, There was no significant differences in the morphology of GerMXene suspensions by increasing the rotation speed up to 3000 rpm for 15 minutes.
Supplementary Figure S28: Morphology characterization of GerMXene at different temperatures from 4 °C to 70 °C. a, SEM images of GerMXene treated at 37 °C for 2 hours. b, SEM images of GerMXene heated at ⁓70 °C for 2 hours. Before these experiments, the material was kept overnight in the fridge at 4 °C. SEM images of GerMXene samples confirmed that storage of GerMXene at different temperatures had no significant effect on its morphology and microstructure.